The invention concerns a discharge lamp, provided with a discharge vessel surrounded, at some distance, by an outer bulb filled with gas and provided with a getter.
Such a discharge lamp is known. An example of such a discharge lamp is a metal halide lamp. In such a known discharge lamp, the outer bulb is often filled with nitrogen, the pressure of which at room temperature is selected to be in the range 250 mbar-600 mbar. The getter is present in the lamp in order to remove hydrogen that comes to be in the outer bulb during lamp manufacture. If this hydrogen is not removed from the outer bulb, this hydrogen also enters the discharge vessel by diffusion through the discharge vessel wall. In this case re-ignition of the discharge lamp will be problematic. In practice it is difficult to find a getter with which in a nitrogen atmosphere small quantities of hydrogen can be removed for the greater part from the outer bulb. The getter must meet the requirement that hydrogen is effectively removed while the getter at the same time must not become poisoned by the nitrogen. The latter requirement often has the consequence that the getter cannot be activated by heating the getter for a certain time at a relatively high temperature. Such activation would increase the xe2x80x9cgetter activityxe2x80x9d for gettering nitrogen to such extent that the getter would become poisoned by nitrogen.
It is an object of the invention to provide a discharge lamp provided with an outer bulb filled with gas and provided with a getter, in which hydrogen is removed in an effective manner from the outer bulb by the getter.
To achieve this a discharge lamp as mentioned in the opening is characterized in accordance with the invention in that the getter comprises more than 80% by weight Zr and Co and moreover one or more elements are chosen from among the rare earth elements.
It has been found that in a discharge lamp in accordance with the invention hydrogen is effectively removed from the outer bulb of the discharge lamp. It was found to be unnecessary to activate the getter, and the getter did not become so poisoned by other gases present in the outer bulb that the hydrogen gettering activity dropped significantly.
In a preferred embodiment of a discharge lamp in accordance with the invention, the gas composition contains nitrogen. It has been found that the getter that is used in the outer bulb of a discharge lamp in accordance with the invention is able to effectively getter hydrogen without becoming saturated with nitrogen and without it being necessary to activate the getter.
Good results have been obtained for embodiments of a discharge lamp in accordance with the invention in which the rare earth metals present in the getter are chosen from the group comprising Ce, La and Nd.
Good results have likewise been obtained for embodiments of a discharge lamp in accordance with the invention in which the percentage by weight of Zr in the getter is selected to be between 75% and 85%, the percentage by weight of Co in the getter between 10% and 20% and the percentage by weight of the rare earth metals between 1% and 10%. Discharge lamps in accordance with the invention with which good results have been obtained are metal halide lamps. It has been found that the quantity of hydrogen in the outer bulb of these lamps after a relatively low number of burning hours has fallen to virtually nil.